Scientists at Argonne National Laboratory have created innovations in the design and fabrication of radio frequency (RF) cavities that improve acceleration of high energy particles: a “cage cavity” that improves cavity performance over solid wall cavities in several ways. The cage cavity improves the vacuum property, reduces power losses due to higher order resonances in the cavity, and allows use of superconducting films instead of bulk metals. Most importantly, the cage cavity is more economical to fabricate and it simplifies the accelerator cryomodule, which also reduces the fabrication costs.

Description

RF cavities are used to produce very high electromagnetic fields to accelerate particles to high energy levels. Using today’s technology, these fields are confined and encased in a metal vessel. As a result, the process of optimizing the design for RF cavities requires a compromise between electromagnetic, mechanical and thermal properties. RF cavities are most commonly constructed of copper or niobium. Niobium cavities are required for high current applications and are difficult and costly to produce. The solid wall design offers only limited access for such essential activities as coupling power, diagnostics, damping, and vacuum pumping. Ports on the cavity body are created to provide access to the cavity’s interior, and usually degrade the RF performance of the cavity.

The cage cavity introduces an alternate design. Instead of using solid wall cavities, a cage cavity is formed by assembling an array of tubes that have the surface contour of the cavity. The spacing between the tubes determines the frequency of the RF fields that can be confined. By selectively bending the tubes into a three-dimensional shape, the Argonne team created a superconducting cage cavity that produces high quality cavity properties, such as resonance frequency, power quality factor, accelerating gradient and efficient cryogenic cooling. In addition, the multi-cell cage cavities can be fabricated to incorporate complex geometries, such as integrating a third harmonic cell into several cells at the fundamental resonance in the cavity.

Benefits

The cage cavity offers significant advantages for superconducting RF (SRF) cavities over conventional designs. In solid-wall normal conducting cavities, holes are cut in the surface to establish ports for power feeds, diagnostics, etc. In SRF cavities, ports cannot be used because the ports degrade cavity performance. Instead, the ports must be placed at the ends of the cavity, resulting in limited functionality because the particle beam cannot be accelerated in those areas. Argonne’s tubular cage design allows for ports to be located anywhere. It offers greater functionality than the solid cavity design. The singularly best advantage of cage cavities is their cost. The costs of fabricating a SRF cavity with tubular cage design are considerably lower, around 10 times less expensive than fabricating a conventional SRF solid wall cavity.

An electron linear accelerator using the cage cavity will lower the cost to install and operate high energy accelerator facilities. The cage cavity accelerator also lowers costs for industrial applications, such as medical isotope production, food sterilization and more.

Applications and Industries

Medical technology

Industrial uses such as polymerization, sterilization and waste treatment

Tubular RF cage field confinement cavityAn RF cavity is provided with a plurality of tubes that are formed into a tubular cage in a predefined shape to define the RF cavity. A selected number of tubes and a selected tube diameter are provided to form a confinement cage for the RF fields within the RF cavity defined by the tubes. The multiple, small metal tubes are selectively bent to form different cavity shapes and sizes as needed to accelerate the particles and function as a confinement cage for the RF fields within the RF cavity defined by the tubes. The cost to fabricate RF cavities using the tubular cage design is significantly lower than the cost of producing a solid cavity using conventional fabrication technology.